Cause Shirou is not right in the head? Also he is his dad, boys like their dad and all but they are still parents before they are friends. And a role model, or just a 'guy friend' would be interesting for Kiyoshi and how he would act on it.
But anyway that was not the point. What I am saying is that Shirou is the least necessary for an interesting dynamic and will also let you get an interesting RP experience out Kiyoshi's interactions with others. Not even approaching how Sakura and Aoi would approach the situation.
You only have 4 slots, so you need to make a call. And honestly, Shirou is the one that brings the less amount of stuff to the RP while causing interesting things for your other choices of characters with his absence.
Also, you do not need to change the sheets much from what you have already written, just alter the stats a little and make sure they have actual weaknesses, it should not take that long since you already have Kiyoshi and Rider done, and Sakura and Shirou as well from 1.0. Not sure about Aoi but didn't you make a sheet for her then to?
Sheets should not take you as long as you make them out to be.